This invention relates to a frequency-modulation (FM) stereo demodulator based entirely on digital signal processing, for use in an FM audio broadcast receiver.
FM stereo demodulators of the switching type, with analog circuitry, have long been used in FM audio broadcast receivers, but as the density of integrated circuits rises, digital circuit configurations are becoming advantageous due to their small size and their compatibility with digital broadcast receiver circuits.
The input to a digital FM stereo demodulator is conventionally a stereo composite signal that has been sampled in synchronization with a sampling clock output by a voltage-controlled oscillator (VCO) or voltage-controlled crystal oscillator (VCXO). If the stereo composite signal is sampled at the right times, it can be demodulated to left-channel and right-channel stereo audio signals simply by selection of certain subsets of the input samples. The functions of a conventional digital FM stereo demodulator accordingly include selecting the relevant input samples, generating an error signal for control of the VCO or VCXO, converting the error signal to an analog signal, and filtering the analog signal for input to the VCO or VCXO. Further details will be given later.
The need for a digital-to-analog converter and analog filter for the error signal is a disadvantage, because these components take up space, and the analog filter in particular cannot easily be integrated with digital signal-processing circuits. The need for an external VCO or VCXO is likewise a disadvantage. The effect of these disadvantages is to increase the parts count and manufacturing cost of the conventional FM stereo demodulator.